Xylem sap flow and stem hydraulics of the vesselless angiosperm Drimys granadensis (Winteraceae) in a Costa Rican elfin forest

For decades, botanists have considered Winteraceae as the least modified descendents of the first angiosperms primarily because this group lacks xylem vessels. Because of a presumed high resistance of a tracheid-based vascular system to water transport, Winteraceae have been viewed as disadvantaged relative to vessel-bearing angiosperms. Here we show that in a Costa Rican cloud forest, stem hydraulic properties, sapwood area- and leaf area-specific hydraulic conductivities of Drimys granadensis L, (Winteraceae) are similar to several co-occurring angiosperm tree species with vessels. In addition, D, granadensis had realized midday transpiration rates comparable to most vessel-bearing trees. Surprisingly, we found that D. granadensis transpired more water at night than during the day, with actual water loss being correlated with wind speed. The failure of stomata to shut at night may be related to the occlusion of stomatal pores by cutin and wax. Our measurements do not support the view that absence of xylem vessels imposes limitations on water transport above those for other vesselled plants in the same environment. This, in turn, suggests that a putative return to a tracheid-based xylem in Winteraceae may not have required a significant loss of hydraulic performance.